Axial turbomachinery noise is prevalent in many products ranging from large scale turbofan engines and compressor/turbine arrays to HVAC systems and computer cooling fans. Noise generated by turbomachinery has both broadband (due to the randomness of turbulent flow and its interaction with blade structures) and tonal components (due to periodic excitation of rotor blades and resonance sources). For subsonic axial fans, broadband noise results primarily from turbulent boundary layer scattering over a blade's trailing edge (TE), tip clearance noise and, potentially, from stall. Tonal noise results from rotor/stator interactions with time-invariant flow distortions and direct field interaction of rotor/stator blades. These tonal noise sources generally radiate axially for ducted fans as a dipole-like source. When spectrally dominant, blade tones are of primary concern in noise control applications due to their particular annoyance. Therefore, robust, cost-effective techniques for reducing their propagation are regularly sought.
Prior approaches used to reduce blade tone sound pressure levels (SPLs) have utilized both active and passive noise control methods. Passive blade alterations, such as rotor/stator spacing, leaning, sweeping or contouring, numbering, and irregular circumferential blade spacing, have been demonstrated effective for fan noise reduction. Also, absorbing liners or other duct cancellation techniques such as Herschel-Quincke tubes can reduce propagations of fan noise within a duct. Obstructions, such as cylindrical rods, can be placed in the near field of a rotor to generate an anti-phase secondary sound field which can then be tuned to reduce blade tone noise. However, difficulty in tuning the response of these interactions often limits their usefulness. Few passive approaches have demonstrated the ability to reduce blade tone noise locally in the blade region with minimal impact on fan efficiency.
Active noise control approaches have been used for blade tone noise reduction, introducing active secondary sources into the existing sound field of an axial fan. Conventional active approaches have used loudspeaker arrays to reduce levels of fan noise propagating down a duct. Due to the associated weight and non-compactness of loudspeakers, piezoelectric actuators have been used more recently as acoustic transducers imbedded into the stator vanes of axial fans to reduce tonal noise propagations. Air injections, either positioned to generate secondary sources through interaction with the rotor blades or used to improve flow non-uniformities generated by a body in a flow field, have been shown to reduce tonal noise. These approaches have proven effective in a laboratory setting, but are generally prohibitively expensive and potentially unreliable in most actual axial fan applications.
The first known implementation of flow-driven resonator source was to generate a canceling sound field that reduced fan noise generated by a centrifugal blower. More recently, a method of using resonators as flow driven secondary sources has been developed for axial fans. This method behaves as a form of active source cancellation wherein fluid flow interacts with a resonator as a means of generating an acoustic source. A single resonator has been shown to be effective for reducing unidirectional propagations of blade tone noise by as much as 24 dB, while an array of resonators equal to the number of stator vanes was used to reduce propagations of both plane-wave and higher order mode propagations by 28 dB.
A fundamental shortcoming of the single resonator axial fan experiments, particularly for plane wave propagations where fan noise radiates as an axially propagating dipole, is that flow driven resonators respond acoustically as monopole sources. For this reason, only unidirectional propagations of the plane wave mode can be reduced using a single resonator or circumferential array of resonators as shown in FIG. 1. While this results in a reduced noise level in one (in this case, downstream) direction, it also may cause an increased noise level in the other (in this case, upstream) direction.